1. Field of the Invention
This invention is related to laser printers having a function of correcting an f.theta. error, and particularly, to an exposing device for the laser printer in which the f.theta. error is corrected without using an f.theta. lens.
2. Description of the Related Art
FIG. 1 shows an exposing section of conventional laser printers. In the laser printer, a laser diode 1 is driven on the basis of image information to emit a laser beam toward a polygon mirror 3 rotating at a constant speed. The polygon mirror 3 reflects the laser beam according to its mirror angle f.theta. so as to scan and selectively expose the surface of a photosensitive drum 2 in a direction shown in FIG. 1. In this exposing operation, an image for one line is formed on the surface of the photosensitive drum 2 as an electrostatic latent image. A dot is formed by driving the laser diode 1 continuously for a period of the dot exposing cycle. A space is formed by inhibiting the driving of the laser diode 1 continuously for a period of the dot exposing cycle. The photosensitive drum 2 is rotated by a preset amount each time the scanning is effected for one line. An image for lines thus obtained is developed as a toner image in which toner is adhered to dot portions, and transferred onto a sheet of paper.
The scanning speed in the main scanning direction varies with the mirror angle f.theta. of the polygon mirror 3. This is a factor which makes the sizes of dots forming an image on the surface of the photosensitive drum 2 to be non-uniform. For example, when the surface of the photosensitive drum 2 is partitioned into four exposure areas A, B, C and D sequentially scanned for 1/4 scanning periods, the width of each dot is relatively large in the exposure areas A and D positioning at end portions of the photosensitive drum 2 and is relatively small in the exposure areas B and C positioning at center portions thereof.
Conventionally, an f.theta. error which thus occurs depending on a mirror angle f.theta. is corrected by means of an f.theta. lens (not shown) which is arranged between the polygon mirror 3 and the photosensitive drum 2 and refracts the laser beam from the polygon mirror 3 according to the mirror angle f.theta. such that the scanning speed is adjusted to be constant. In this correction technique, however, the total number of components is increased and a space for mounting the f.theta. lens is required. Therefore, there is a problem that it is difficult to attain a laser printer of a small size and a low cost.
As another correction technique, a technique which does not use an f.theta. lens as stated above and controls the dot exposing cycle during the scanning of exposure areas such as the areas A, B, C, and D in the main scanning direction is known. In this technique, the dot exposing cycle for each dot is set to be shorter in the scanning of the exposure areas A and D than in the scanning of the exposure areas B and C. As shown in FIG. 2, the period of the dot exposing cycle is set equal to a period corresponding to three print clock pulses in the scanning of the exposure areas A and D, and to a period corresponding to four print clock pulses in the scanning of exposure areas B and C, for example. Accordingly, the width of each dot can be uniform among the exposure areas A, B, C, and D. Further, an f.theta. error can be more accurately corrected if the surface of the photosensitive drum 2 is partitioned into more exposure areas than four and the dot exposing cycle is variably set according to each of the exposure area.
Meanwhile, some of the conventional laser printers have an edge correction function. These printers irradiate a laser beam onto part of the spaces which have a width for one dot and located next to edge dots in the main scanning direction so that a smooth outline can be obtained in a printed image. In a case where the period of the dot exposing cycle is set equal to a period corresponding to four print clock pulses and an outline of inclination H shown in FIG. 3A is required for edge dots G, F, and E, the laser diode 1 is additionally driven during the periods corresponding to three print clock pulses, two print clock pulses, and one print clock pulse in scanning the spaces next to the edge dots G, F, and E, respectively. This allows the outline of inclination H to have reduced steps along the dots G, F, and E. Therefore, the outline can be smooth or linear.
In addition, some of the conventional laser printers have a variable tone setting function. In these printers, a laser beam is not continuously irradiated for a period of the dot exposing cycle equal to a period corresponding to a preset number of print clock pulses. Instead, these printers inhibits driving of the laser diode 1 during the period corresponding to a part of the preset number of print clock pulses. In a case where the period of the dot exposing cycle is set equal to a period corresponding to four print clock pulses, driving of laser diode 1 is inhibited during the period of at least one pulse which is selected from the first to fourth print clock pulses according to a tone level to be set for one dot. For example, when driving of the laser diode 1 is inhibited during the period of the second print clock pulse shown in FIG. 4A, the tone level of the dot can be lowered in comparison with the case where the laser diode 1 is driven during the entire periods of first to fourth print clock pulses.
However, it is difficult for the correction technique of controlling the dot exposing cycle to be applied to laser printers having an edge correction function or a variable tone setting function described above.
In the laser printer having the edge correction function, normal edge correction is interfered by the control of the dot exposing cycle. For example, the laser diode 1 is additionally driven in a state where the period of the dot exposing cycle is set equal to a period corresponding to four print clock pulses in the scanning of the exposure areas A and D shown in FIG. 1 and equals to a period corresponding to eight print clock pulses in the scanning of the exposure areas B and C shown in FIG. 1. In this case, as shown in FIG. 3B, an outline of inclination H would be obtained in the exposure areas A and D and an outline of inclination I would be obtained in the exposure areas B and C. That is, edge correction cannot be performed uniformly among these exposure areas to obtain outlines of the same inclination.
Further, in the laser printer having the variable tone setting function, normal variable tone setting is interfered by the control of the dot exposing cycle. For example, the laser diode 1 is inhibited to be driven during the period of one print clock pulse in a state where the period of the dot exposing cycle is set equal to a period corresponding to four print clock pulses in the scanning of the exposure areas A and D shown in FIG. 1 and equals to a period corresponding to eight print clock pulses in the scanning of the exposure areas B and C shown in FIG. 1. In this case, as shown in FIG. 4B, the width of an unexposed portion would be narrower in the exposure areas B and C than in the exposure areas A and D. That is, variable tone setting cannot be performed uniformly among these exposure areas to obtain the same tone level.